“sleepover at Sannes house” – there should be an apostrophe somewhere in Sannes. The house belongs to him/her after all.

“Sanne and me became friends” – grammatically, it should be Sanne and I. In any case, you’ve left out talking marks, and this is what Sophie is saying, right?

“Sanne and him” – I’d say “he and Sanne” sound better.

“It's not gone like I had expected.” – That’s unusual wording.

I know it’s technically Sophie talking, but she seems old enough to conform to grammar. And, considering her state, she should be focusing a little more on her words than she normally would.

I don’t think the page breaks work too well. Since they’re not flashbacks, it works better if you just had the quotation marks showing where Sophie was talking. Would get rid of the confusion of changing perspectives as well.

"It's... it's nothing, really. Why am I telling you this?" Zach snapped at hearing this words. Of course, way too quickly. – you should split that into two different paragraphs, since otherwise it sounds like Zach is the one who said the sentence until the next line where we realise it was Sophie.

“She's too good to abuse that, although she has considered so once or twice.” – “considered doing so” I think.

“"What do you mean?" I heard a slight tone of hesitance in her voice, confusing me. She shook her head.

"No... no, it's nothing."”

The page break should be before “she shook her head” not after. It confuses who’s speaking where otherwise.

“Sannes” – again, apostrophe – “Sanne’s.

There’s a few repetitions – no point listing them all.

Zach is such a good older brother. Now I want one again. Sadly, no siblings here.

Have no idea why this hasn't been reviewed before, but this is freaking good! Man, I sound like a squealing teenage girl now. XD But, I just love this story, and how well, confused, Sophie is about everything, and the positive brother-sister relationship between Zach and her (you hardly see any positive sibling relationships, these days, as a trend in fiction, but you're really good at those) and the first person narration sections. You should write like that more often, because it gives everything this suspense and immediacy - and I like it!

Anyhow,

I think I've nearly read all your English works, by now, except for the Destiny one and I still have to catch up on Rooting for Romance (looking forward to that even though I'm so behind on updates)